Shape memory alloys (SMA) form a group of metals that have interesting thermal and mechanical properties. If a SMA component is deformed while in a martensitic state (low yield strength condition) and then heated to its transition temperature to reach an austenitic state, the SMA component will resume its original (undeformed) shape. The rate of return to the original shape depends upon the amount and rate of thermal energy applied to the component. When heat is removed from the component, it will return to the martensitic state in which the component can again be deformed.
A known SMA rotary actuator is described in U.S. Pat. No. 5,127,228. The device of the '228 patent includes two concentric tubular SMA members. The inner SMA member is twisted relative to the outer SMA member, and the ends of both members are mechanically restrained to an indexed position. One of the SMA members is provided to generate mechanical torque clockwise and the other member counter-clockwise. Each SMA member has a dedicated heater connected to an electric power supply. Initially, both SMA members are in a martensitic state.
To operate the '228 device, electrical power is applied to the heater of an SMA members to cause that member to transition from martensitic to austenitic state. Upon rotation to a desired rotational position by one SMA member, electrical power is discontinued to that member and is applied to the other SMA member. This generates torque in the opposite direction. The application of force in both rotational directions thus appears to provide a means of holding the output of the actuator in a particular fixed position and providing rotation in both directions.
The '228 device suffers from a number of disadvantages. The most cumbersome aspect is that the '228 device requires two SMA components. To maintain a specific loaded rotational position, electrical power must be continuously applied to the heater elements of each SMA member. Both of these attributes add system weight and complexity as well as require excessive power. The '228 device is also problematic in that it requires a well considered design so that the heaters do not heat the wrong SMA member and thereby unintentionally create an actuator malfunction.
Other known SMA rotary actuators utilize one rather than two SMA members to provide rotation. These devices use the SMA member to provide rotation in one direction, while using a conventional spring to provide rotation of the actuator in the other or return direction. SMA rotary actuators which use conventional springs are limited in the scope of their application, since the force generated by conventional springs is limited. Thus, for such actuators to be utilized for large force application, the springs would need to be large. This adds considerable weight and bulk to the actuator mechanism. Such actuators also require electrical power to either the SMA member or heater elements to continuously maintain the SMA member in a fixed rotational position. Another limitation of these actuators is the fact that conventional springs deteriorate over time, which limits the reliability of the actuator.
Therefore, a need exists for a SMA rotary actuator which can provide either low or high amounts of torque, operate in both directions of rotation using a single SMA member, be capable of maintaining a desired position of rotation upon removal of heat from the SMA member, and be capable of rotation in the opposite direction without the application of electrical power. In an ideal arrangement, the rotary actuator would be capable of generating a very large rotation torque over a large angle, be capable of locking in a desired position, be capable of returning to the neutral (or zero) position upon removal of electrical power or disengagement of a lock and would not utilize conventional springs solely for the counter (return to neutral) force. To provide for installation in limited space locations, the actuator should also be small in size and have low weight.